FULLERANES, FULLERENES AND THE INTERSTELLAR EXTINCTION

Three features of the extinction vary with position in the interstellar medium in a similar way: they are all strong in the diffuse medium but are weak or absent in dense clouds. It is proposed that they are different facets of a single "variable component" of the extinction and that the carriers are fullerane molecules, such as the family C60H(m)(m = 0,1,...,60). The opacity of the fulleranes is expected to be modulated under the chemical control of the environment and this is proposed as the explanation of the pattern of variation of the extinction: the opacity will be high in the diffuse medium, where the molecules bear few hydrogen atoms on account of the harshness of the conditions, and low in dense clouds, where the molecules are more heavily hydrogenated. The extinction of the variable component is weak in the red but rises rapidly into the ultraviolet, and the spectra of laboratory samples of C60 and C70 are broadly similar. Roughly a quarter of the interstellar abundance of carbon is required in the fulleranes to match the intensity of the extinction and, since the efficiency with which these molecules can be produced in the carbon arc is high, a pyrogenic origin in carbon stars and planetary nebulae is indicated. Features in the optical spectrum of C70 show a resemblance to the very broad structure on the extinction curve, both in wavelength and in amplitude.